Christo, the lead author of the paper and a postdoctoral researcher at the university, stated that this new krypton dating technique can locate and date ice that is over one million years old. The oldest ice discovered so far comes from ice cores approximately 800,000 years old. Through this technology, researchers can search for old ice in other regions with the goal of tracing polar ice back to 1.5 million years ago. This is very exciting because many interesting aspects of Earth's climate occurred before 800,000 years ago, which we currently cannot study in ice core records.
Radioactive krypton isotopes possess ideal geophysical and chemical properties for the absolute dating of water or ice samples that have dissolved atmospheric gases. It is similar to carbon-14 dating, which measures the decay of radioactive isotopes at a constant rate. The difference is that krypton is an inert gas that does not undergo chemical reactions and has a more stable half-life of 230,000 years.
Krypton is produced by cosmic rays bombarding the Earth and is then stored in air bubbles trapped within Antarctic ice. It has one radioactive isotope (krypton-81) that decays very slowly, and one stable, non-decaying isotope (krypton-83). By comparing the ratio of the stable isotope to the radioactive one, the age of the ice can be determined.
Although scientists have been interested in radioactive krypton isotope technology for over forty years, the number of krypton-81 atoms is so limited that they were difficult to count. It was not until 2011 that a breakthrough in detection technology made the identification of krypton-81 atoms feasible. The new atom counter, named Atom Trap Trace Analysis (ATTA), was developed by a team led by nuclear physicist Lu Zheng-Tian at the Argonne National Laboratory near Chicago.
According to a recent report by Phys.org, this work was funded by the U.S. National Science Foundation and the U.S. Department of Energy. In experiments at the Taylor Glacier in Antarctica, researchers collected 300-kilogram blocks of ice, melted them in containers, and stored the released air bubbles in bottles. The krypton was isolated from the air at the University of Bern in Switzerland and then sent to the Argonne Laboratory to count the krypton-81 atoms. Researchers determined that the Taylor Glacier sample was 120,000 years old based on the isotope ratio, and verified this by comparing it with measurements of methane and oxygen in the atmosphere from ice cores of the same period.
Researchers stated that the atom trap is quite sensitive and can capture and count individual atoms. The only problem is that there is not much krypton in the air, and therefore not much in the ice, which is why such large sample volumes are required for melting.
The current challenge is to find some of the oldest ice in Antarctica, which may not be as easy as it sounds. Edward Brook, a geologist at Oregon State University and co-author of the paper, said, "Most people think it's just a matter of drilling deeper into the ice core, but it's not that simple. Very old ice may exist in isolated patches at the bottom of the ice sheet, but in many places, this ice may have already melted into the ocean. Of course, there are some special areas where old ice is exposed at the edge of an ice field."
Researchers say that reconstructing Earth's climate back 1.5 million years is important because changes in the frequency of ice ages occurred during the so-called Mid-Pleistocene Transition. Over the past 800,000 years, Earth is believed to have entered an ice age approximately every 100,000 years, but there is evidence suggesting that the frequency of this transition was every 40,000 years prior to that time.